A method of welding a plurality of steel sheets in a stack includes spot welding method. In the spot welding method, a plurality of steel sheets are stacked and sandwiched between a paired electrodes, and then electrical current is applied between the paired electrodes under pressure along the rods of the paired electrodes. The electrical current resistive-heats to melt the sandwiched portions in an area having a predetermined diameter and then the melted portions solidify, and thus a melted and solidified portion referred to as weld nugget is formed (Japanese Patent Laid-Open No. 2005-211934).
In recent years, steel sheets high in tensile strength have been used as the materials for body of automobiles, and steel sheets high in tensile strength is welded by spot welding to achieve both the weight reduction and the safety in automobiles. Two methods represent investigation methods for the spot welding strength. The first method welds the stacked edge portion of the two rectangular steel sheets followed by applying tensile forces in the directions along the surface planes of the two plates. The second method welds the crossed two rectangular steel sheets at crossing center followed by applying tensile forces in the directions rectangular to the surface plane of the two plates at both ends of each of the steel sheets. The first method is referred to as the TSS (Tensile Shear Strength) test, and the second method is referred to as the CTS (Cross Tension Strength) test.
Regarding a carbon steel excellent in both ductility and toughness, a dual-phase steel having fine crystal grains has been investigated, and the segregation of carbides at the grain boundary is known as an effective mean (Japanese Patent Laid-Open No. 2007-332457). Although the increased carbon content in the material is required to segregate carbides at the grain boundary, as high carbon content makes the spot welding zone too hard, the brittle spot welding zone cause a problem that the weld strength remarkably decreases. So, the carbon content is controlled to be about 0.15 mass % or less in the steel sheets for automobiles widely used.
Among the methods of spot welding the steel sheets, the method first conducting low-frequency electrical current application and then conducts the high-frequency electrical current application is known (WO 2011/013793).
Further, Japanese Patent No. 4006513 and the paper: Characterization of Microstructure for Tempered Martensite by Atomic Force Microscopy; Masao Hayakawa, Saburo Matsuoka, Materia Japan, Vol. 43, No. 9, pp. 717-723, 2004 disclose the method for evaluating the fine dual-phase microstructure of the high-strength steel by Atomic Force Microscopy (AFM). Note that the evaluated sample is the steel type of JIS-SCM440 with carbon content of 0.4% by mass.